1. Field of the Invention
The invention relates to a method and apparatus for finding the location of a pointing instrument on a tablet.
2. Description of the Related Art
U.S. Pat. No. 4,210,775 discloses a method and apparatus for producing an electrical signal which represents the location of a pen with respect to a grid of spaced conductors. The pen is provided with a coil for producing an alternating magnetic field in response to an alternating current electrical signal of a predetermined magnitude and frequency that is applied thereto, thereby inducing a plurality of alternating current signals in the spaced conductors. A sensing unit scans sequentially the spaced conductors in the X and Y directions. A position counter executes a counting operation while the sensing unit is performing a scanning operation. The sensing unit includes a differential amplifier circuit which is coupled sequentially to the spaced conductors so as to receive the respective alternating current signals in the spaced conductors. The output of the differential amplifier circuit is fed to a phase sensitive detector. When a phase reversal is detected on opposite sides of the pen, the operation of the position counter is stopped, and the output of the position counter is recorded. The recorded digital output of the position counter represents the location of the pen with respect to the grid of spaced conductors.
It has been shown that in the conventional digitizing apparatus, the position counter is operated from the start of each counting operation until a phase reversal condition is detected. Referring to FIGS. 1A and 1B, plot (A') shows the filtered output of the phase sensitive detector 102, while plot (B') shows the operating period 104 of the position counter. From these drawings, the following drawbacks can be observed:
1. The operating period of the position counter is relatively long. PA1 2. The size of the position counter should be relatively large so as to generate a precise output. PA1 3. The size of the position counter should be increased correspondingly when the sample area is increased.